1. Field of the Invention
The present invention relates to an image forming apparatus and method and, more particularly, to an image forming apparatus and method for performing image formation using a recording head having an array of a plurality of recording elements.
In particular, the present invention relates to an apparatus comprising a mechanism for automatically adjusting printing characteristics of a recording head of an ink-jet recording apparatus, and is particularly suitable for an apparatus for forming a multi-gradation color image by overlaying ink droplets.
2. Related Background Art
Along with an increase in popularity of information processing equipment such as copying machines, wordprocessors, computers, and the like, and communication equipment, apparatuses for performing digital image recording using a recording head based on an ink-jet system or a thermal transfer system have increasingly become popular as image forming (recording) apparatuses for such equipment. Such a recording apparatus normally employs a recording head constituted by integrating a plurality of recording elements (also referred to as a multi head hereinafter) so as to increase recording speed.
For example, of ink-jet recording heads, a so-called multi-nozzle head constituted by integrating a plurality of ink discharging orifices and liquid channels is normally used. In a thermal head of a thermal transfer system or a thermal system, a plurality of heaters are normally integrated.
However, it is difficult to manufacture uniform recording elements in a multi head due to variations of characteristics caused by manufacturing processes, variations of characteristics caused by head constituting materials, and the like. Thus, the characteristics of the recording elements suffer from variations to some extent. For example, in the multi-nozzle head, the shapes of the discharging orifices and liquid channels suffer from variations, and in the thermal head, the shapes of heaters, resistors, and the like also suffer from variations. Such nonuniform characteristics of the recording elements appear as nonuniform sizes and densities of recorded dots, resulting in density nonuniformity in a recorded image.
In order to solve this problem, various methods of visually finding density nonuniformity, or visually checking an adjusted image, and manually correcting signals to be supplied to the recording elements to obtain a uniform image have been proposed.
For example, the following manual correction means is known. That is, in a multi head 330 having an array of recording elements 331, as shown in FIG. 26A, if density nonuniformity is visually found, as shown in FIG. 26C when uniform signals are input to the recording elements, as shown in FIG. 26B, the input signals are corrected, as shown in FIG. 26D, so that large input signals are supplied to the recording elements corresponding to a low-density portion, and small input signals are supplied to the recording elements corresponding to a high-density portion.
As is well known, in a recording system capable of modulating a dot size or a dot density, recording elements modulate recording dot sizes in accordance with an input to attain multi-gradation recording. For example, in an ink-jet recording head based on a piezo system or a system utilizing thermal energy, drive voltages or pulse widths of signals applied to discharging energy generating elements such as piezo elements, electricity-heat converters, and the like are modulated according to an input signal. In a thermal head, drive voltages or pulse widths of signals to be applied to heaters are modulated according to an input signal. By utilizing these operations, it is possible to make uniform dot sizes or dot densities of the recording elements, thereby obtaining a uniform density distribution, as shown in FIG. 26E.
In some cases, it is impossible or difficult to modulate the drive voltages or pulse widths, or it is difficult to adjust a density over a wide range even when the drive voltages or pulse widths are modulated. In this case, for example, when one pixel is constituted by a plurality of dots, the number of dots to be recorded is modulated in accordance with an input signal, so that a large number of dots can be recorded for a low-density portion, and a small number of dots can be recorded for a high-density portion. When one pixel is constituted by one dot, the number of ink discharging operations (the number of injections) for one pixel can be modulated to change a dot size. With these operations, the density distribution can be uniform, as shown in FIG. 26E.
In order to solve the above-mentioned problem, the following means is effective. That is, a density nonuniformity reading unit is arranged in an image forming apparatus. A serviceman or a user places a test pattern recorded by using a head on the reading unit, and a density nonuniformity distribution in a recording element array range is periodically read to regenerate density nonuniformity correction data. With this means, even when the density nonuniformity distribution of the head is changed, the correction data is regenerated accordingly. Therefore, a uniform image free from a nonuniformity can be maintained. As such image recording apparatus, see U.S. Pat. No. 5,038,208 and U.S. patent applications Ser. No. 07/480,041 (filed Feb. 14, 1990) and Ser. No. 07/516,129 (filed Apr. 27, 1990).
However, in such as arrangement, the following problems are left unsolved.
A nonuniformity measurement test pattern formed by a recording head is used in density nonuniformity reading. When the relationship between a reading range by the reading means and the size of the test pattern, or the positional relationship therebetween is not proper, light reflected by a background portion of a recording medium located outside the edge portion of the test pattern is incident on a reading sensor, or there is a problem of incidence of reflected light. As a result, precise density reading cannot be performed.
Upon correction, when the test pattern is recorded in a state wherein there is a recording element which cannot perform a normal recording operation (e.g., a discharging orifice suffering from a discharging error in an ink-jet recording head), the density nonuniformity cannot be correctly recognized due to the presence of such a recording element.